Acetaldehyde is one of the main precursors for the discoloration of polyethylene terephthalate. Simple and more complex chemical reactions involving participation of .beta.-hydroxyethyl groups on chain ends or created by polymer chain scission e.g. (by hydrolysis) are major routes to formation of acetaldehyde thermal dehydration of ethylene glycol and diethylene glycol is also known to lead to formation of acetaldehyde.
Acetaldehyde can then undergo aldol condensation upon heating in the presence of a catalyst. Further condensation of acetaldehyde can occur to form a conjugated structure which may be a principal chromophore in polyethylene terephthalate.
Because of the effect acetaldehyde imparts to PET articles and substances contained therein, it has been attempted to remove the acetaldehyde from the PET resin. One known method of removal is by heating the resin under reduced pressure, but has been found insufficient in removing all the acetaldehyde.
Another known method consists of further polymerizing the prepolymer at a temperature of from 270.degree. to 290.degree. C. for less than thirty minutes to achieve an intrinsic viscosity of about 0.4 to 0.6 and then further polymerizing the reaction product of step one in the solid state to provide a stable resin with an intrinsic viscosity of at least 0.7.
All intrinsic viscosity numbers hereinafter referred to as I.V., were measured in a 60/40 phenol/tetrachloroethane mixed solvent at 30.degree. C.
One known method involves a process whereby during thermal processing of saturated polyesters in the molten state, gaseous oxygen is excluded and as a result, acetaldehyde formation is minimized. This involves a process in which all gaseous elemental oxygen is excluded from contacting the polymer, including the step of purging oxygen containing gases from the atmosphere in contact with the polyester by means of an inert gas, before raising the temperature of the polyester to the molten condition and physically working the polyester.
Another known process relates to the removal of acetaldehyde from crystalline PET granules by passing a dry inert gas through a bed of resin at a temperature of 170.degree. C. to 250.degree. C. for a time sufficient to remove the acetaldehyde. This process is conducted prior to extrusion of the resin.